1. Field of the Invention
The invention relates to a system in which communication is performed between a programmable logic controller (PLC) and other devices, and when a communication line failure has occurred, the location of the failure can be identified.
2. Discussion of Background
A programmable logic controller (hereinafter referred to as “the PLC”) operates by communicating with other devices. Regarding a PLC, Japanese Patent Application Publication No. 6-324723 (JP 6-324723A) describes that in the case where one station is to receive data from another station, if the data is unable to be received for a predetermined period of time and a timer reaches a set time, it is determined that a communication failure has occurred. Japanese Patent Application Publication No. 2-235104 (JP 2-235104A) describes that the number of times retried calling operation has been performed is displayed to determine the number of retries that have been required for establishing data exchange communication.
However, if many communication devices are connected, it is difficult to identify a failed portion in communication lines.